How Were the First Mountains and Valleys Formed?
The first mountains and valleys were the crust formed as a result of the cooling of the molten mass of the earth. As the planet contracted the crust twisted and cracked, forming new mountains. Mountains are still being formed by volcanic eruption. A crack in the earth’s crust allows molten rock and ash to be forced out, forming a cone-shaped mountain growing as it continues to erupt.
Fault-mountains are formed when the earth’s crust cracks, or faults, under pressure from inside, and one side of the break is pushed up against the other to form a cliff. The highest mountains are in the Himalayas where some are over 25,000 feet. Only in the Rocky Mountains and in the Andes are there any others over 20,000 feet.
Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth’s crust (tectonic plates). Folding, faulting, volcanic activity, igneous intrusion and metamorphism can all be parts of the orogenic process of mountain building. The formation of mountains is not necessarily related to the geological structures found on it.
The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics. From the late 18th century until its replacement by plate tectonics in the 1960s geosyncline theory was used to explain much mountain-building.
A valley is a low area between hills or mountains often with a river running through it. In geology, a valley or dale is a depression that is longer than it is wide. The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys. Most valleys belong to one of these two main types or a mixture of them, at least with respect to the cross section of the slopes or hillsides.
A valley formed by flowing water, called fluvial valley or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a bottom. Shallower slopes may produce broader and gentler valleys. However, in the lowest stretch of a river, where it approaches its base level, it begins to deposit sediment and the valley bottom becomes a floodplain.
A valley carved by glaciers is normally U-shaped and resembles a trough. This trough valley becomes visible upon the recession of the glacier that forms it. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice. Floor gradient does not affect the valley’s shape, it is the glacier’s size that does. Continuously flowing glaciers – especially in the ice age – and large-sized glaciers carve wide, deep incised valleys, sometimes with valley steps that reflect differing erosion rates.